1. Field of the Invention
The invention relates in general to a polysilicon film and a method of forming the same, and more particularly to a polysilicon film having a smooth surface and to a method of forming the same.
2. Description of the Related Art
The techniques for manufacturing thin film transistor (TFT) on a glass substrate include the amorphous silicon (a-Si) process and the low-temp polysilicon (LTPS) process. The major differences between the a-Si process and the LTPS process are their complexity and the electrical characteristics of the manufactured devices. The LTPS TFT possesses higher mobility, but the process for fabricating the LTPS TFT is more complicated than that for fabricating the a-Si TFT.
Several methods, including the excimer laser annealing (ELA) method, the continuous grain silicon (CGS) method, the continuous wave (CW) laser method, the sequential lateral solidification (SLS) method and the metal induced lateral crystallization (MILC) method, have been developed to convert the amorphous silicon layer into a polysilicon film. A laser beam, such as one produced by an excimer laser, a continuous wave (CW) laser, or abeam pulse laser, can be used for irradiating the amorphous silicon layer. Recently, the beam pulse laser is commonly used, and the method of lateral crystallization by causing a temperature gradient is also popular.
Taking the sequential lateral solidification (SLS) method for example, the use of optical phase shift masks that have different transparency can cause a lateral temperature gradient in an amorphous silicon layer so as to induce lateral grain growth.
No matter what polysilicon film is used for fabricating the metal-oxide-semiconductor (MOS) device or TFT device, the surface roughness of the polysilicon film has a significant effect on the electrical reliability and uniformity of the device. Using the conventional laser annealing methods to convert the amorphous silicon into the polysilicon grains usually produces protrusions (or tips) on the surface of the polysilicon film, and therefore roughens the surface. The root mean square (RMS) roughness of the polysilicon film formed by the conventional laser annealing methods is in a range of approximately 7-9 nm. FIG. 1 shows a SEM (scanning electron microscope) image of a polysilicon sample annealed by the ELA method. The SEM image indicated that the height of the protrusions on the polysilicon surface is in the range of about 1000 Å to 1200 Å. FIG. 2 is perspective view, in the form of an AFM (atomic force microscope) image, of a polysilicon sample also annealed by the ELA method. The image of FIG. 2 clearly shows that a lot of protrusions project from the polysilicon's surface.
Therefore, a method for forming a polysilicon film having a smooth surface to improve the electrical reliability and uniformity of devices is desirable.